This invention relates to shared medium access networks, such as satellite, LMDS, UMTS, cable modem or fibre in the loop access networks, in particular to fibre to the home (FTTH). The following description relates to FTTH, but it will easily be seen how it applies to other scenarios with similar characteristics. FTTH networks can be made more economic by sharing fibre facilities and head end equipment across a number of customers. Passive Optical Networks (PONs) fall into this category. In such a network, a single head end node, normally physically located on the network provider's premises, connects to a number of customer located outstations via a passive optical splitter (POS) which provides a fanout to (typically) 16 outstations.
Traffic transmitted in the downstream direction (from the head end to the outstations) appears at all outstations and is selected by a given outstation based on an address included in a header associated with each data packet. In the upstream direction a multiple access protocol is used to ensure that only one outstation transmits information at a time.
Such networks can be used to transmit multiple services to a customer, including video services and data services. On the customer premises an Optical Network Unit (ONU) connects to the fibre network and provides one or more interfaces to which the customer can attach end user equipment. This equipment might include one or more Set Top Boxes (STBs) for interfacing video services to a television set and one or more personal computers. Each of these devices could connect via, for example, an Ethernet interface.
The ONU will normally be supplied by the network operator who can control the software included within the ONU itself. Devices attached to the Ethernet interfaces, however, are often outside the control of the network operator and the end user may therefore be able to load software which is outside the control of the network operator.
Video services consist of television channels which can be selected for viewing by individual end users and can be classified into two categories: multicast and Video on Demand (VOD). Multicast video channels are viewed simultaneously by a number of users. Such channels may include, for example, standard broadcast channels, subscription channels (where the user pays a monthly fee for the right to view the channel whenever he wants) and pay per view channels (where the user pays to view a particular programme). VOD channels are programmes requested by a particular user and supplied only to that user. Each VOD channel requires a dedicated data path from a video server within the network. Multicast channels avoid dedicated paths from the server to each user by including multicasting features in the data path, typically using a router situated at the head end of the access network. When the first user requests a multicast channel, that channel is delivered to the head end router from the server and a connection is made through the router to the access network. If another user subsequently requests to view the same channel, a second connection is made within the router to cause the channel to be sent out on the interface to which the second user is connected. Since the second user is joining an existing channel, no additional data capacity is required on the link between the server and the router. Protocols exist for signalling from an end user device to a router to join and leave a multicast group. When the data transmission is based on Internet Protocol (IP), a multicast signalling protocol known as Internet Group Management Protocol (IGMP) may be used. Conventionally in IP networks, a multicast stream is given a destination IP address drawn from a group of addresses reserved for multicast IP packets. Similarly, when using Ethernet as the medium access control (MAC) layer, the destination MAC address is drawn from a group of addresses reserved for multicast Ethernet frames. Thus at both the IP layer and the MAC layer, the address used represents the content of the multicast data stream rather than identifying a specific destination.
An algorithm for mapping IP layer multicast addresses to MAC layer multicast addresses is given in the Internet Engineering Task Force (IETF) Request for Comment (RFC) 1112. This is a many to one mapping where a single MAC address could represent many different schemes. In systems using this mapping, the multicast channel cannot be identified uniquely at the MAC layer and the IP layer destination address must be checked to guarantee uniqueness.
In a variation of the multicast protocol, known as source specific multicast (SSM), both the source IP address and the destination IP address are required to identify uniquely a specific multicast stream. In a system using SSM the destination multicast MAC address is not guaranteed to be unique. Since current protocols do not reflect the source IP address in the source MAC address, SSM channels cannot be uniquely identified at the MAC layer and the source address at the IP layer must be checked.
A problem arises when the end user connection is a shared medium network (such as a PON): a multicast stream will be delivered to the ONUs situated on the premises of all end users on the PON whenever one of the users requests that stream and, by listening to traffic on that address, a second user would be able to view the service even though he may not have paid to receive it. This could lead to loss of revenues to the content provider which is highly undesirable.